1. Field of the Invention
The invention relates to a fluid actuated positioner or servo device for converting a relatively low pressure fluid signal into a magnified mechanical force, particularly for use in a marine control system.
2. Prior Art
For many years pneumatic or hydraulic controls have been used in the marine industry for control of many devices, for example engine throttle controls, gear box/clutch controls and numerous other applications. Commonly the signal is a varying pressure pneumatic signal, or a direct mechanical movement from a manual control. The signal, by itself, would commonly have insufficient force to operate the apparatus to be controlled. The force must be magnified, and commonly this has been done using pressurized hydraulic fluid through a servo device. Many devices are available for this purpose, a particular device being found in the present inventor's U.S. Pat. No. 3,496,833 issued 1970. This device uses a mechanical input signal which is magnified by a pressurized hydraulic servo device. More typical devices are found in U.S. Pat. Nos. 3,757,640 (Karol); 3,502,001 (Moore) and 4,089,252 (Patel et al). Some of these patents disclose devices in which input force is mechanical or fluid actuated, and most of them use the force multiplying effects of pressurized hydraulic fluid acting on a piston/cylinder combination.
In general, many of the devices of the prior art perform satisfactorily in some situations but are not easily adaptable to the wide range of applications required in marine situations. Specifically some devices of the prior art require highly accurate manufacturing tolerances, thus are costly to produce with corresponding high maintenance expense. Also, close tolerances produce difficulties in alignment of the device in a typical installation, and sometimes difficulties can arise in installations where the apparatus cannot tolerate a wide range of input forces without a corresponding loss of accuracy. In other instances, a change in output force requires a proportionately greater change in input forces to "unstick" a valve, etc., and this tends to "over shoot" the desired setting. Thus the change in input signal is exagerated and then the control must be backed off in an attempt to obtain the desired setting. Some devices also experience "creep" and cannot maintain a desired setting for any length of time.